On some aircraft there can develop a problem known as "blue ice". Blue ice typically results when the blue dye rinse water used in many aircraft toilets leaks out to locations at or near the skin of the aircraft and freezes while the aircraft is in flight. There is a danger that as the blue ice builds up on the skin of the aircraft that it eventually may break off and be ingested into an aircraft engine. In addition, blue stains may be formed on the fuselage and ramp area when the aircraft is on the ground.
On aircraft having gravity feed toilets, the waste holding tank located beneath each toilet is emptied by maintenance personnel on the ground via an evacuation pipe connected between the bottom of the holding tank and the skin of the aircraft. There is typically a first valve located in the evacuation pipe near the outlet from the holding tank and a second valve located near the skin of the aircraft. When both of these valves leak, blue water is permitted to reach the skin of the aircraft and freeze.
A number of waste water holding systems have been disclosed. For example, U.S. Pat. Nos. 4,275,470 and 4,357,719 both by Badger et al disclose an aircraft vacuum flush toilet which uses a vacuum (ambient pressure) to aid in opening a flush valve and pressure (cabin pressure) to hold the flush valve closed. Furthermore, U.S. Pat. No. 4,332,041 by Kristoffersen discloses a vacuum toilet system which uses a vacuum pump to suck waste from a toilet bowl into a waste holding tank and a blower to pressurize the waste holding tank to blow waste out of the tank when being serviced.